1. Field of the Invention
There are a number of previously known proximity detection systems for detecting the proximity between an object and a transmitting antenna. For example, such systems can be used on vehicle gates and the like to prevent the vehicle gate from contacting a vehicle, person or other object as the gate is moved from a raised to a lowered position.
2. Description of the Prior Art
Many of these previously known proximity detection systems utilize an oscillator which transmits a high frequency signal to the antenna. Proximity between an object and antenna changes the capacitance of the antenna and thus the frequency of the oscillator. The change of frequency of the oscillator is detected by other circuitry which then generates an appropriate output signal.
These previously known proximity detection systems, however, have not proven wholly satisfactory in use. One major disadvantage of these previously known systems is that factors other than proximity between the antenna and an object causes changes in the oscillator frequency and thus false triggering of the output signal. For example, radio frequency interference,, component degradation, ambient temperature and the like can all cause changes in the oscillator frequency and result in false triggering of the system.
A still further disadvantage of these previously known proximity detection systems is that it was difficult, if not impossible, to adjust the sensitivity of the system. Thus, it was not possible to adjust the closeness or proximity between the antenna and the object before triggering of the system would occur. However, in many situations, it is desirable to trigger the output signal caused by proximity between the antenna and the object at relatively great distances, for example several feet, while, conversely, in other situations it is desirable to trigger the system when there is very close proximity, for example a few inches, between the antenna and the object.